diff --git a/plugins/mail/nullmailer_queue b/plugins/mail/nullmailer_queue
index a5028f3f..04aceb3c 100755
--- a/plugins/mail/nullmailer_queue
+++ b/plugins/mail/nullmailer_queue
@@ -54,7 +54,7 @@ GPLv2
 
 =cut
 
-. $MUNIN_LIBDIR/plugins/plugin.sh
+. "$MUNIN_LIBDIR/plugins/plugin.sh"
 
 queuedir=${queuedir:-/var/spool/nullmailer/queue}
 errordir=${errordir:-/var/spool/nullmailer/failed}